Southern French Alpes

Champsaur is on the Route Napoleon, the path Napoleon took when he ended his exile in Elba and returned to France. So there should be sites memorializing this historic return in the area.

I am more familiar with the eastern side of Hautes Alpes, but I can say that the area is spectacular, with easy access to isolated valleys. You can drive up the Valgaudémar valley, with several waterfalls along the way, and options for short hikes. It is also easy to get to the lonely hamlet of Prapic, and up a side valley towards Champoléon.
The road to/from Grenoble (route Napoléon) is quite spectacular, too.

Further south, Gap is worth no more than a quick stop in the old town, but Serre-Ponçon lake is worth an outing. If the water is low, I recommend going to see the submerged viaduct at Chanteloube - and it's a good spot to access the lake in any case. You could also get spectacular lake views from the top of the chairlift at Réallon if it is running when you're there. If you're willing to drive a bit further, you can check out the cathedral town of Embrun, the Vauban citadel of Mont-Dauphin, and the Boscodon abbey and forest above Embrun.

Thank you both!

A place I forgot to mention: Col du Noyer, up a very minor (and slightly hairy) road from the Champsaur, with great views over the area and a lovely refuge for a drink or meal.